A methodological approach was developed to study oxygen reduction reaction in double perovskites materials of series LnBa_1-xSr_xCo_yFe_1-yO_5+δ (Ln=Gd, Pr) for solid oxide fuel cell (SOFC) electrode. Anisotropy in the layered structure is attributed for higher oxygen diffusivity in the Ln plane as compared to the Ba plane. Molecular dynamics simulations on PrBaCo₂O_5+δ (PBCO) calculated the oxygen anion diffusion coefficient in the a-b direction to be 3 x 10^-8 cm²s^-1 at 873 K which was observed to be higher than in the c plane (D=8x10^-9 cm²s^-1 at 873K, Figure 1a).The electrochemical performance was observed to correlate with the oxygen diffusion coefficient [1],[2]. Recently, experimental studies have reported PrBa_0.5Sr_0.5Co_2-xFe_xO_5+δ (PBSCF) to be the most promising cathode material for SOFC, measuring high peak power density. The diffusivity in PBSCF (1.18 x 10^-7cm²s^-1) was calculated to be higher by an order of magnitude as compared to PBCO (3x10^-8 cm²s^-1 at 873 K, Figure 1a) [1]. In order to understand the relative control of surface reaction and bulk diffusion, Dense thin-film of electrodes of PBSCF (2 µm thick) with <500 nm grain sizes was deposited using spray pyrolysis. The polarization resistance of PBSCF thin-film was calculated to be 7.21 Ω cm² at 873 K, (Figure 1b), which was ascribed to the bulk-diffusion in the electrode. In parallel, Density Functional Theory (DFT) calculations were utilized to calculate the surface (ϒ) and oxygen vacancy (E_ov) formation energies. The oxygen vacancy formation in the Gd plane (Eov= 98.4 kJ/mol) of GBCO was measured to be lower than that of Ba plane (E_ov=266.3 kJ/mol), therefore; vacancies were observed to be concentrated in the Gd plane. The surface energy of the Ba plane was measured to be minimum ϒ =7.2 kJ/mol, which makes it the most exposed surface, while it is least diffusive for oxygen anions. Therefore, larger size bulk microstructures of GBCO were measured to give lesser than expected oxygen anion diffusivity (D=6.04x10^-10cm²s^-1). GBCO particles of reduced size are expected to expose higher energetic Gd plane, which may enhance diffusivity. The average particle size was observed to be reduced to 20 nm in case of bio-milling synthesis of GBCO leading to a 20% improvement in the ASR of electrode [3] (Figure 1c). MD simulation on a GBCO nanoparticle showed, the diffusion in nanoparticles to represent two distinct regimes; one corresponding to the surface and other the bulk. The stability of nanoparticles were validated by calculating the root mean square deviation (RMSD) of atomic positions in MD simulations. The thickness (20A⁰) of diffusive surface shell regime was observed to be particle size independent, while the thickness of the diffusive core regime varies with the size of nanoparticle (Figure 1d). The diffusivity of the shell regime (D=3.61x10^-9 cm² s^-1) was calculated higher than of the core (D=5.07x10^-10 cm² s^-1at 1073K). Thus, nanoparticles showed improved electrochemical performance as compare to the bulk electrode.

 Reference:

1. Uzma Anjum, M. Ali. Haider, et.al “Solid State Ionics”, 280, 24 (2015)
2. Uzma Anjum, M. Ali. Haider, et.al “Int.J. Hydrogen Energy, 41, 7631 (2016)
3. Baishakhi Mazumder, et.al. “ J. Materials Chemistry, 17, 3910, (2007)